[Issue 23998] @mustuse should require opCast(T:bool) is checked

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23998

--- Comment #3 from Dennis <dkorpel at live.nl> ---
@mustuse is supposed to prevent discarding values of that type. I'm not a fan
of tacking on additional rules about `opCast` to a feature that should be
simple and orthogonal. It looks like what you really want is type state, which
Timon Gehr has been advocating for, but that would require a DIP.

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